This invention relates to a system for providing emergency power on an aircraft. More specifically, this invention provides a stored energy subsystem using aviation fuel and compressed air which is burned in a fuel rich mixture to yield a controlled, high temperature, high pressure gas. The gas produced by the system may then be used to drive a turbine wheel, which may provide power to drive such devices as engine starters, electrical generators, and hydraulic pumps.
Generally, an aircraft has one or more primary engines which provide thrust for the aircraft, as well as pressurized bleed air for the environmental control systems. The primary engine also provides power to drive electric generators and hydraulic pumps, both of which are necessary for powering instruments and flight control systems. In addition, many aircraft also have an auxiliary power engine to provide electric and hydraulic power, as well as bleed air to the aircraft when the primary engines are not operating, for example when the aircraft is on the ground. The auxiliary power engine may also provide power to start the primary engines either on the ground or in flight. Both the primary engine and the auxiliary power engine operate on aviation fuel from the aircraft's main fuel tanks, mixed with air drawn from the atmosphere as the combustion components. For maximum fuel efficiency, these engines operate in a stoichiometrically air rich, fuel lean mode. In many instances, starting the auxiliary power engine requires an external power source such as a ground based start cart, a pressurized air tank, or an emergency power system. Since the auxiliary power engine is primarily designed to operate on the ground where the air is relatively dense, the auxiliary power engine may be incapable of operating at higher elevations, for example above 55,000 feet. It is therefore evident that in many applications the auxiliary power engine would not be able to restart a failed primary engine above 55,000 feet, and in this event there would be no electrical or hydraulic power available. Also, since both the primary engine and the auxiliary power engine operate on fuel from the main fuel tanks, if this fuel supply is depleted there will be no source of power for the electrical and hydraulic power systems to allow the pilot to control and land the aircraft.
It is therefore desirable to have on an aircraft an emergency power system capable of operating independent of external conditions which can provide emergency electrical and hydraulic power to the flight control systems and may be used to restart the auxiliary or primary engines. These are the minimum requirements of the emergency power system. Since they are only operated in the event of an emergency, these systems remain stored and inactive for long periods of time, but are required to start instantly and provide continuous power output for a prespecified duty cycle. Ideally, such an emergency power system would be compact, lightweight, highly reliable, easily maintained, require no special handling of materials or fuels, while providing a combustion process which is controllable and which produces a clean, non-toxic combustion gas.
Presently, emergency power units primarily rely on liquid hydrazine based fuels sprayed into a catalyst bed to generate a pressurized gas. These units are in use on several aircraft and combine high performance with low weight. However, liquid hydrazine is highly corrosive and toxic, thereby requiring special handling procedures and design considerations. The catalyst material is expensive, and when the catalyst is depleted it must be replaced. Further, the combustion gas which is produced is toxic and therefore limits ground testing of the emergency power unit.